Migraine is a severely disabling headache disorder for which treatment is ineffective for millions of Americans, because individuals have no reliable way to predict drug responses or side effects. These shortcomings stem in large part because the root cause of migraine is not fully understood, even though a substantial body of research has revealed neuronal, vascular, genetic, and environmental clues. It is well established, however, that altered neuronal excitability is inherent to migraine symptoms, and understanding the mechanism behind this increased excitation is the focus of the proposed work. Our preliminary data show a rise in cerebrospinal fluid (CSF) sodium concentration ([Na+]) during migraine in humans and we found that the [Na+] also increased in the brain, CSF, and eyes in a rat migraine model. Moreover, we found rats in this model have increased sensitivity to touch and light that are similar to migraine symptoms. From these results and current published physiology, we propose a mechanism to explain how brain [Na+] rises excessively in migraine from overactive Na+, K+ -ATPase transporters (NKATs) in brain capillary endothelial cells (BCECs). Our theory predicts that BCEC NKAT activity above an upper limit elevates [Na+], increasing neuronal excitability that causes the headache and extreme sensitivity to light, sound, smells, and motion. We will test this theory in the rat migraine model by answering three complementary questions. 1. Does [Na+] rise in brain, CSF, and eyes at the beginning of the behavioral effects, and does migraine medication reverse the changes? We will measure brain and CSF [Na+] and glutamate in vivo using the unique 21 Tesla MRI at the National High Magnetic Field Laboratory (Tallahassee, FL). 2. Does rising [Na+] that perfuses the normal brain, brain lining, and retina increase neuronal excitability? We will study how the firing of neurons in the rat brain and eyes responds to changes in [Na+] using electrophysiology. This should let us know in what regions nerves are more sensitive to [Na+] and whether the locations that are most sensitive are the same as those areas found to change in the MRI studies. 3. Does BCEC NKAT activity lead to the rise of [Na+] in the brain, intracranial CSF, and eyes? We will compare the timing and locations of the behavioral effects with BCEC NKATs in the rat model. We will determine if BCEC NKAT expression/activity changes in regions known to be involved in migraine and, if so, does it change before other known markers of migraine, including glutamate. We will test if the anti-migraine medications sumatriptan and telcagepant reverse the [Na+] changes, and if this correlates with decreased BCEC NKAT expression/activity. Verification of our theory that BCEC NKAT overactivity elevates [Na+], increases neuronal excitability, and responds to current migraine therapies will provide a mechanism to evaluate and develop new treatments, since NKATs have many regulators. Objective in vivo biomarkers of sodium and glutamate will guide further research of pathophysiology in the animal migraine model, and will justify efforts to apply this to humans.